1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, a printer, a multifunction peripheral, or the like, a photosensitive member has a surface that is uniformly charged by a charging device and is optically scanned by an optical scanning apparatus to thereby form an electrostatic latent image on its surface in response to image information. Thereafter the electrostatic latent image is developed by a developing device using toner as a developing agent and is visualized as a toner image. The toner image is transferred onto a sheet with a transfer device and fixed onto the sheet by heating and pressure by a fixing device. A series of image forming operations is completed by discharge from the apparatus of the sheet with the toner image fixed thereto.
Conventionally, an optical scanning apparatus which is a principal constituent component of an image forming unit incorporated into an image forming apparatus such as a copying machine, a printer, a multifunction peripheral, or the like has a housing including an internal space having a predetermined volume. This housing accommodates optical components and the like such as a scanning optical system or the like including a plurality of reflective mirrors that return constant-speed scanning light and guide the light to the photosensitive member, such as an optical beam generating device that generates optical beams (laser light) (optical beam emission unit), a polygon mirror formed by a hexagon or the like that deflects the light beams emitted from the optical beam generating device, a polygon motor that rotates and drives the polygon mirror (polygon scanner motor), a control board that mounts electronic components including an IC and supports the polygon motor, and an imaging lens (fθ lens) that images (converts to constant-speed scanning light) the optical beams deflected by the polygon mirror onto the photosensitive member.
However the polygon motor mounted on the control board produces heat when driven. The inner portion of the housing is heated by the heat produced by the polygon motor and therefore the various components disposed in the housing inner portion are affected by the heat. Therefore a heat strategy is typically adopted in an optical scanning apparatus. For example, a method of cooling the inner portion of the housing has been disclosed in which a draft air duct for enabling air flow is formed in the housing to thereby allow heat in the inner portion of the housing to escape to the outside through the draft air duct.
Alternatively, an optical scanning apparatus in which a single deflection device is disposed in a center of the housing to thereby deflect and allocate the optical beams in two symmetrical directions with the deflection device has been proposed.
Alternatively, an optical scanning apparatus has been proposed in which two housings (optical chambers) are disposed in parallel and scan respective light beams in two symmetrical directions about the deflection device. According to this optical scanning apparatus, since the size of one of the housings is reduced, positional deviation of the scanning line is effectively suppressed and problems associated with color shift are suppressed. Alternatively, the effect on color shift caused by the return mirror can be reduced by making the number of return mirrors take a value of one.
However the optical beam generating device, the polygon motor, the optical components and the like are disposed in the housing. When a draft air duct is formed randomly as an indentation with respect to the above type of housing, there is a risk that localized thermal deformation may increase as a result of large change in the shape of the housing from a square shape, and that the installation region for the optical beam generating device, the polygon mirror, the optical components, and the like will be subjected to a high degree of warping. When warping occurs in the installation region for the optical beam generating device, the polygon mirror, the optical components, and the like in the housing, those components become inclined and light beams can no longer be suitably guided.
Alternatively, since the above configuration requires that two light beams scan in one direction, the two light beams must be separated and guided to the photosensitive member and therefore a space with respect to the height direction in the housing must be ensured. Consequently, problems arise in relation to the increased size of the housing. Alternatively, a large width must be ensured in a height direction of the imaging lens provided for constant-speed scanning of the two light beams, and this causes cost increases. The size of the housing is further increased due to the fact that four optical systems for scanning four light beams are all contained in a single housing.
Alternatively, in the above optical scanning apparatus, since the optical path only returns once from the deflection device to the photosensitive member, there is the problem that there is an increase in the width in the height direction of the housing. In particular, when an imaging lens with a long focal length is used, the housing size is increased since the height of the housing increases proportional to the focal length.
Consequently, the thermal deformation amount of the housing due to temperature fluctuation increases, the scanning position of optical beams deviates and therefore tends to result in problems in relation to color shift.